In Situ Analysis of Historical Preservation Fluids in Sealed Containers with Spatially Offset Raman Spectroscopy

Understanding and monitoring of preservation fluids is essential for ensuring the long-term integrity of fluid-preserved specimens in natural history collections. However, analytical characterization remains challenging, particularly as noninvasive and portable solutions are required. This study presents the first in situ application of spatially offset Raman spectroscopy (SORS) combined with multivariate analysis for classifying preservation fluids in historical biological specimens. A total of 46 fluid-preserved samples from the Natural History Museum, London, UK were analyzed under collection conditions using a portable device. The method accurately identified the preservation fluids in 78.5% of cases and showed partial agreement in another 15%, often with visually similar or chemically complex solutions. Only 3 samples (6.5%) were misclassified or unclassified. SORS spectra revealed main excipients and low-concentration additives, with classification ambiguities typically arising from fluorescence effects or secondary components not included in the calibration set. Additionally, the proposed approach allowed distinguishing between different types of glass (and/or plastic) container providing potential insights into fluid-container interactions and historical storage conditions. Overall, the approach demonstrated high chemical specificity, sensitivity to mixtures, and broad potential for conservation and collection management.